A. Field of the Invention
The present invention relates to a power semiconductor device and, more specifically, to a super-junction (hereinafter may be abbreviated as SJ) MOSFET.
B. Description of the Related Art
A MOSFET has been developed which broke through the characteristic limit of conventional silicon MOSFETs by employing, as a drift region, what is called a super-junction structure (hereinafter may be referred to as “p-type/n-type column structure” or “SJ column structure”). A super-junction structure is a collection of column-shaped p-type and n-type regions that are arranged in parallel and in close contact with each other on a high-impurity-concentration (hereinafter referred to as low-resistivity) semiconductor substrate and that extend perpendicularly to its major surface. Mass-production of an SJ-MOSFET initially used a manufacturing method called a multi-stage epitaxial method to realize the super-junction structure. The multi-stage epitaxial method is as follows. An epitaxial layer to serve as a drift layer is grown on a low-resistivity semiconductor substrate in several steps. Patterning and ion implantation are repeated in such a manner that p-type regions and n-type regions having fixed patterns are formed in the epitaxial growth stages of the respective layers, whereby the p-type regions and the n-type regions are connected to each other in the direction perpendicular to the major surface. In this manner, a super-junction structure is formed as a collection of p-type and n-type column-shaped regions that are arranged parallel with each other and extend perpendicularly to the major surface. However, this method requires a long, complex manufacturing process and hence the manufacturing cost and the chip cost are high.
On the other hand, in recent years, a buried-trench SJ-MOSFET has been developed which can reduce the manufacturing cost. This type of SJ-MOSFET is manufactured in the following manner. A wafer is formed in which an n-type epitaxial layer is grown on a low-resistivity n-type semiconductor substrate. Trenches are formed at prescribed intervals, so as to penetrate through the n-type epitaxial layer and reach the low-resistivity n-type semiconductor substrate, by performing etching from the wafer front side (in some cases, trenches are formed so as not to completely penetrate through the n-type epitaxial layer and hence not to reach the substrate). Then, the trenches are filled in completely by causing p-type epitaxial layers to grow in the respective trenches, whereby a p-type/n-type column structure is formed. The manufacturing process of this method is shorter and simpler than that of the above-described multi-stage epitaxial method, and hence this method may reduce the manufacturing cost.
In versions of the buried-trench epitaxial method that have been developed to date, MOS gate structures each of which consists of p-type base regions, n-type source regions, a gate oxide film, a channel region, etc of a MOSFET are formed after formation of a p-type/n-type column structure. However, a phenomenon occurs that the impurities in the p-type and n-type columns move by diffusion due to thermal history that is necessary for formation of the MOS gate structures. If the p-type or n-type impurity in each column diffuses into other columns (mutual diffusion), the net doping concentration (i.e., the difference between the p-type doping concentration and the n-type doping concentration) of each column decreases. To compensate for this phenomenon, it is necessary to set the p-type and n-type impurity dopes higher in advance (otherwise the on-resistance becomes high). This increases the absolute values of the variations of the impurity dopes, which leads to a problem that resulting large variations in breakdown voltage lower the breakdown-voltage-related yield. The above problem of mutual diffusion is unavoidable also in the above-described multi-stage epitaxial method which is already in the mass-production stage, and is one of reasons why the breakdown-voltage-related yield of SJ-MOSFETs generally is not very high.
FIG. 2A is a schematic sectional view of an SJ column structure for illustration of the above-described impurity mutual diffusion phenomenon. As shown in FIG. 2A, an SJ column structure which is a collection of p-type regions 3 and n-type regions 2 is formed on low-resistivity n-type semiconductor substrate 1. The solid line in FIG. 2B indicates a net doping concentration profile (which is step-like at the pn junction), taken across the cross section, of a portion indicated by an arrow in FIG. 2A in the case where the SJ-MOSFET is not subjected to any thermal history after formation of the SJ column structure. The broken line in FIG. 2B indicates a net doping concentration profile in the case where the SJ column structure is subject to thermal history and in view of reductions in doping concentrations due to mutual diffusion the SJ column structure is given higher impurity concentrations in advance so that the total net doping of p-type region 3 and n-type region 2 are made equivalent to those of the solid-line curve of FIG. 2A due to mutual impurity diffusion that is caused by the thermal history. Since the total net doping of p-type region 3 and n-type region 2 indicated by the solid line are equivalent to those indicated by the broken line, the breakdown voltage obtained when a reverse bias is applied in the case where the net doping concentration profile is as indicated by the solid line is approximately equal to that in the case where the net doping concentration profile is as indicated by the broken line. (More strictly, because of the difference between the two net doping concentration profiles, different space charge profiles occur when the regions concerned are depleted. Therefore, a small difference exists between the electric field strength profiles and hence a small difference occurs between the breakdown voltages, each of which is the integral of an electric field.)
Furthermore, the on-resistances of the two cases are approximately identical. In the case of an n-channel MOSFET, the carriers are electrons and hence the resistance of n-type region 2 (one layer) will be calculated below. In the case of the step-like profile indicated by the solid line in FIG. 2B, the electric conductance σ1 of the one-layer n-type region 2 is given byσ1=s0Dqρn0μn/l(Ω−1)  (1)where s0 is the width of n-type region 2 across its cross section, D is the depth of n-type region 2, l is the height of n-type region 2, q is the amount of the elementary electric charge, ρn0 is the n-type net doping concentration, and μn is the electron mobility.
In the case of the profile indicated by the broken line in FIG. 2B in which mutual diffusion is taken into consideration, the electric conductance σ2 of the n-type region 2 is given byσ2=∫qρn(s)μnDds/l(Ω−1)  (2)where ρn(s) is the net doping concentration distribution along the cutting line, and s is the position on the cutting line. The integration is done over the width of the n-type region 2. If the mobility is constant, Equation (2) is modified as follows:σ2=qμnD∫ρn(s)ds/l  (3)
Since the total net doping of n-type region 2 in the case with mutual diffusion is equivalent to that of the case without mutual diffusion, a relationshipDl∫ρn(s)ds=s0Dlρn0  (4)holds. From equations (3) and (4), we obtainσ2=qμnDs0ρn0/l=σ1.  (5)
That is, the on-resistance in the case with mutual diffusion is equal to that in the case without mutual diffusion. However, in actuality, although the total net doping is the same, the total doping concentration (i.e., the sum of the p-type doping concentration and the n-type doping concentration) increases due to the mutual diffusion and hence the mobility decreases a little (the mobility depends on the total doping concentration). Therefore, the resistance of n-type region 2 is a little increased by the mutual diffusion.
As described above, even if mutual impurity diffusion occurs between p-type regions (columns) 3 and n-type regions (columns) 2 because the SJ column structure is subjected to thermal history, the on-resistance/breakdown voltage tradeoff is hardly deteriorated. However, this is true only under ideal conditions that the concentrations of introduced impurities have no variations.
In practice, the p-type and n-type impurity concentrations vary due to variations in a manufacturing process. For example, assume a junction, having a step-like profile, of a p-type region (column) and an n-type region (column) each of which has an impurity concentration of 1×1015 cm−3. If it is assumed that the variation of each impurity concentration due to variations in a manufacturing process is ±10%, that is, ±1×1014 cm−3, in the worst case the p-type concentration becomes 1.1×1015 cm−3 and the n-type concentration becomes 0.9×1015 cm−3; the charge balance between the p-type region and the n-type region is calculated as 1.1/0.9=122%. This charge imbalance lowers the breakdown voltage.
Next, consideration will be given to the case in which there is mutual diffusion. For example, assume that the doping concentration of each of the p-type region (column) and the n-type region (column) is decreased by 1×1015 cm−3 by the mutual diffusion (since the doping effects of the pair of dopants, that is, the p-type dopant and the n-type dopant, cancel each other out, the decrease in the doping concentration of the p-type region (column) is equal to that in the doping concentration of the n-type region (column)). It is necessary that the concentration of each of the p-type region (column) and the n-type region (column) before the SJ column structure be subjected to thermal history be set at 2×1015 cm−3 (step-like profile). If it is assumed that the variation of each impurity concentration due to variations in a manufacturing process is ±10%, that is, ±2×1014 cm−3, and that the doping concentration is decreased by 1×1015 cm−3 by the mutual diffusion, in the worst case the concentration of the p-type region (column) becomes 1.2×1015 cm−3 and the concentration of the n-type region (column) becomes 0.8×1015 cm−3; the charge balance between the p-type region (column) and the n-type region (column) is calculated as 1.2/0.8=150%. This charge imbalance lowers the breakdown voltage to a large extent. As is understood from the above discussion, in the case with mutual diffusion, the influence of variations in a manufacturing process is amplified when it is intended to obtain the same electrical characteristics. The breakdown-voltage-related yield is thereby lowered.
In the manufacturing method of the buried-trench SJ-MOSFET, the factors causing variations in the impurity concentrations of the p-type regions (columns) and the n-type regions (columns) include variations of the impurity concentration of the n-type epitaxial regions, the impurity concentration of the p-type buried epitaxial regions, the trench width, and the trench taper angle. These factors cause a charge imbalance between the p-type regions (columns) and the n-type regions (columns). If the thermal history is made more severe, the influence of the above variations becomes more serious. One method for preventing the breakdown-voltage-related yield from being lowered even if the thermal history is made more severe is to set the original impurity concentrations of the p-type regions (columns) and the n-type regions (columns) low. However, these impurity concentrations cannot be set low because doing so increases the on-resistance. Therefore, it can be said that it is desirable to decrease the number of thermal history events that cause mutual diffusion after formation of the p-type regions (columns) and n-type regions (columns).
In connection with the manufacturing method of the above-described buried-trench SJ-MOSFET, US-A1-2003-0008483 (corresponding to JP-A-2002-83962) is known which discloses a manufacturing method of an SJ-MOSFET which employs a super-junction structure in which a drift region is a collection of column-shaped p-type and n-type regions that are arranged in parallel and in close contact with each other on a low-resistivity semiconductor substrate and extend perpendicularly to its major surface. Furthermore, in this manufacturing method, deterioration of the characteristics is prevented by decreasing the number of thermal history events to which the super-junction structure is subjected in post manufacturing steps.
However, according to the disclosure of US-A1-2003-0008483, in the manufacturing method of an SJ-MOSFET, in the case where the breakdown voltage rating is 600 V, the thickness (in the direction perpendicular to the major surface of the substrate) of the SJ column structure that is necessary for attaining such a breakdown voltage is about 50 μm. An SJ-MOSFET is manufactured by forming MOS gate structures on the front side, decreasing the wafer thickness to about 50 μm by grinding the back surface, and finally forming an SJ column structure. However, such a thin wafer is very prone to break and hence the yield tends to be low.
If an SJ-MOSFET is manufactured by using a thick wafer (more than 350 μm in thickness) without grinding the back surface of a wafer unlike in the above method, the trench width needs to be 6 μm and the trench depth becomes at least 350 μm. No practical manufacturing technique for forming such deep trenches stably by etching has been established yet. In addition, in the above-mentioned method, strictly positioning patterns on two surfaces requires a precision double-side mask aligner.
Furthermore, as for the trench etching from the back surface, it is necessary to stop the etching accurately at the bottom portions of the surface p-type base regions (Xj depth: 3 to 5 μm). The breakdown voltage decreases in either case of the etching depth being too small or too large. This is problematic in that the breakdown-voltage-related yield tends to be low. The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.